What is an Optical Filter? (And, what makes a good one?)
Filters allow wavelengths of interest pass through unadulterated, and absorb or reflect the rest.
The following concepts and terms are key to recognizing a good filter:
Peak Transmission is the maximum transmission of the filter (generally given as a percentage), telling you how much of the desired wavelength reaches the detector or your eye.
Blocking (OD) refers to the unwanted wavelengths that are absorbed or reflected.
Full-width half-max (FWHM) describes the width of the transmitting region at half of the maximum transmission value (nm). Because of the physics of thin-film interference filters, FWHM generally increases with wavelength.
Signal to Noise Ratio
Types of Filters
There are a variety of filters useful for astrophotography and naked eye viewing.
Filters that can distinguish between oxygen and hydrogen can reveal hotter and cooler spots in a nebula. These can also reduce the effects of light pollution.
Neutral Density Filters
Enhance lunar and planetary observations.